Ford Motor and Volkswagen are progressing toward a potential agreement to join forces on self-driving cars and have overcome earlier obstacles, according to people familiar with the talks. Breakthroughs in the deliberations Thursday include a possible framework for Volkswagen to work with and invest in Argo AI, the Ford-backed autonomous-vehicle startup, said the people, who asked not to be identified because the talks were private. The automakers discussed an approximate valuation for the company of $4-billion, one of the people said.